Method and apparatus for efficient signaling and usage of resources for wireless communications supporting circuit switched and packet switched sessions

ABSTRACT

Method and apparatus efficiently signal and use resources for wireless communications supporting circuit switched (CS) and packet switched sessions (PS). Signaling and interaction between the wireless transmit/receive unit (WTRU), and various network entities, such as the Mobility Management Entity (MME), the Visitor Location Register (VLR), and Base Stations (BS), are used to implement circuit switched fall back (CSFB) in a PS system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Nos.61/320,027, filed Apr. 1, 2010; 61/303,761, filed Feb. 12, 2010;61/293,084, filed Jan. 7, 2010; 61/265,917, filed Dec. 2, 2009; and61/256,421, filed Oct. 30, 2009, the contents of which are herebyincorporated by reference herein.

BACKGROUND

It has been a goal in wireless communications to devise a mechanism toenable a wireless transmit/receive unit (WTRU) to place or receive acommunication during an ongoing session without service interruption.For example, in a Third Generation Partnership Project (3GPP) Long TermEvolution (LTE) scenario, a goal has been to devise a mechanism toenable a WTRU, while it is connected to a packet switched (PS) domain ofan Evolved Universal Terrestrial Radio Access Network (E-UTRAN), toplace (i.e., originating) or receive (i.e., terminating) circuitswitched (CS) services or vice versa. Examples of CS services mayinclude at least such services as a CS voice call, a short messageservice (SMS), location services, or supplementary services. Theinitiation of a CS session during an ongoing PS session by a WTRU isreferred to as CS fallback (CSFB).

SUMMARY

Method and apparatus for efficient signaling and usage of resources forwireless communications supporting circuit switched (CS) and packetswitched (PS) sessions are disclosed. The behaviors of various wirelessnetwork entities, including a Mobility Management Entity (MME), aVisitor Location Register (VLR), and Base Stations (BSs), and thewireless transmit/receive units (WTRUs) that communicate with thenetwork, are defined in order to deal with issues that arise byimplementing circuit switched fall back (CSFB) in a Packet Switched (PS)system.

A VLR may decide to abort a CSFB procedure for a mobile terminating (MT)CSFB call to a wireless transmit/receive unit (WTRU), and send an abortmessage to an MME, in order to avoid an inter-system change for a WTRU.The MME may send a Service Reject message to the WTRU indicating thatthe CS call is aborted, and ignore incoming messages pertaining to thecancelled call. Bearer contexts may be synchronized between atransmitter entity and a receiver entity by including bearer contextstatus information in any Non-Access Stratum (NAS) message transmittedto the receiver entity.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1A is a system diagram of an example communications system in whichone or more disclosed embodiments may be implemented;

FIG. 1B is a system diagram of an example wireless transmit/receive unit(WTRU) that may be used within the communications system illustrated inFIG. 1A;

FIG. 1C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 1A;

FIG. 2 is a block diagram of a tracking area update (TAU) responsemessage including an Evolved Packet System (EPS) Update resultInformation Element (IE);

FIG. 3 is a flow diagram of a method for including the EPS bearercontext status in any NAS message;

FIG. 4A is a flow diagram for a procedure between a WTRU and an MMEwhere there is an SGs Interface between the MME and an MSC/VLR;

FIG. 4B is a flow diagram for a procedure performed by a WTRU when if isinformed that the CS domain is unavailable;

FIG. 5A is a flow diagram of a procedure in response to a WTRU placingan emergency call;

FIG. 5B shows a flow diagram of a procedure for reducing delay in theCSFB procedure for emergency calls;

FIG. 6 is a flow diagram of a procedure when the WTRU performs aninter-system change when a location area has changed;

FIG. 7 is a flow diagram of procedure for a TAU being triggered inresponse to a CSFB failure;

FIGS. 8A and 8B show example CSFB procedures for two CSFB calls with andwithout enhancements, respectively;

FIG. 9 is a flow diagram of a procedure for avoiding an inter-systemchange for a WTRU when a CS that triggered a MT CSFB is cancelled; and

FIGS. 10A and 10B are flow diagrams of examples of two possiblescenarios where race conditions may occur between CSFB and intra-LTEHandover.

DETAILED DESCRIPTION

The teachings described herein may be applicable to any accesstechnology that may support circuit switched (CS) or packet switched(PS) sessions. Examples include, but are not limited to, Long TermEvolution (LTE), Wideband Code Division Multiple Access (WCDMA), GlobalSystem for Mobile communications (GSM), General Packet Radio Service(GPRS), Enhanced Data rates for Global Evolution (EDGE), Code DivisionMultiple Access (CDMA2000), IEEE 802 technologies such as 802.11, 802.16and Worldwide Interoperability for Microwave Access (WiMAX), UniversalMobile Telecommunications System (UMTS), or any future technology. Forpurposes of explanation, the various embodiments are described in an LTEcontext, but the various embodiments may be implemented using anytechnology that may support CS and/or PS sessions.

FIG. 1A is a diagram of an example communications system 100 in whichone or more disclosed embodiments may be implemented. The communicationssystem 100 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 100 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems100 may employ one or more channel access methods, such as code divisionmultiple access (CDMA), time division multiple access (TDMA), frequencydivision multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrierFDMA (SC-FDMA), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a radioaccess network (RAN) 104, a core network 106, a public switchedtelephone network (PSTN) 108, the Internet 110, and other networks 112,though it will be appreciated that the disclosed embodiments contemplateany number of WTRUs, base stations, networks, and/or network elements.Each of the WTRUs 102 a, 102 b, 102 c, 102 d may be any type of deviceconfigured to operate and/or communicate in a wireless environment. Byway of example, the WTRUs 102 a, 102 b, 102 c, 102 d may be configuredto transmit and/or receive wireless signals and may include userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,consumer electronics, and the like.

The communications systems 100 may also include a base station 114 a anda base station 114 b. Each of the base stations 114 a, 114 b may be anytype of device configured to wirelessly interface with at least one ofthe WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to one or morecommunication networks, such as the core network 106, the Internet 110,and/or the networks 112. By way of example, the base stations 114 a, 114b may be a base transceiver station (BTS), a Node-B, an eNode B, a HomeNode B, a Home eNode B, a site controller, an access point (AP), awireless router, and the like. While the base stations 114 a, 114 b areeach depicted as a single element, it will be appreciated that the basestations 114 a, 114 b may include any number of interconnected basestations and/or network elements.

The base station 114 a may be part of the RAN 104, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals within aparticular geographic region, which may be referred to as a cell (notshown). The cell may further be divided into cell sectors. For example,the cell associated with the base station 114 a may be divided intothree sectors. Thus, in one embodiment, the base station 114 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 114 a may employ multiple-inputmultiple output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, infrared (IR), ultraviolet (UV), visible light, etc.). Theair interface 116 may be established using any suitable radio accesstechnology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104 and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 116 using wideband CDMA (WCDMA). WCDMAmay include communication protocols such as High-Speed Packet Access(HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed DownlinkPacket Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 114 a and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface116 using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1X, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, and the like. In oneembodiment, the base station 114 b and the WTRUs 102 c, 102 d mayimplement a radio technology such as IEEE 802.11 to establish a wirelesslocal area network (WLAN). In another embodiment, the base station 114 band the WTRUs 102 c, 102 d may implement a radio technology such as IEEE802.15 to establish a wireless personal area network (WPAN). In yetanother embodiment, the base station 114 b and the WTRUs 102 c, 102 dmay utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, etc.) to establish a picocell or femtocell. As shown in FIG. 1A,the base station 114 b may have a direct connection to the Internet 110.Thus, the base station 114 b may not be required to access the Internet110 via the core network 106.

The RAN 104 may be in communication with the core network 106, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. For example, the core network 106may provide call control, billing services, mobile location-basedservices, pre-paid calling, Internet connectivity, video distribution,etc., and/or perform high-level security functions, such as userauthentication. Although not shown in FIG. 1A, it will be appreciatedthat the RAN 104 and/or the core network 106 may be in direct orindirect communication with other RANs that employ the same RAT as theRAN 104 or a different RAT. For example, in addition to being connectedto the RAN 104, which may be utilizing an E-UTRA radio technology, thecore network 106 may also be in communication with another RAN (notshown) employing a GSM radio technology.

The core network 106 may also serve as a gateway for the WTRUs 102 a,102 b, 102 c, 102 d to access the PSTN 108, the Internet 110, and/orother networks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) andthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired or wireless communications networks ownedand/or operated by other service providers. For example, the networks112 may include another core network connected to one or more RANs,which may employ the same RAT as the RAN 104 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities, i.e., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks. For example, the WTRU 102 c shown in FIG. 1A may be configured tocommunicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram of an example WTRU 102. As shown in FIG. 1B,the WTRU 102 may include a processor 118, a transceiver 120, atransmit/receive element 122, a speaker/microphone 124, a keypad 126, adisplay/touchpad 128, non-removable memory 106, removable memory 132, apower source 134, a global positioning system (GPS) chipset 136, andother peripherals 138. It will be appreciated that the WTRU 102 mayinclude any sub-combination of the foregoing elements while remainingconsistent with an embodiment.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In another embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, UV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and receive both RF and light signals. It will be appreciatedthat the transmit/receive element 122 may be configured to transmitand/or receive any combination of wireless signals.

In addition, although the transmit/receive element 122 is depicted inFIG. 1B as a single element, the WTRU 102 may include any number oftransmit/receive elements 122. More specifically, the WTRU 102 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 102 mayinclude two or more transmit/receive elements 122 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 106 and/or the removable memory 132.The non-removable memory 106 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 1C is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the core network 106.

The RAN 104 may include eNode-Bs 140 a, 140 b, 140 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 140 a, 140 b, 140c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 140 a, 140 b, 140 c may implement MIMO technology. Thus,the eNode-B 140 a, for example, may use multiple antennas to transmitwireless signals to, and receive wireless signals from, the WTRU 102 a.

Each of the eNode-Bs 140 a, 140 b, 140 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 1C, theeNode-Bs 140 a, 140 b, 140 c may communicate with one another over an X2interface.

The core network 106 shown in FIG. 1C may include a mobility managementgateway (MME) 142, a serving gateway 144, and a packet data network(PDN) gateway 146. While each of the foregoing elements are depicted aspart of the core network 106, it will be appreciated that any one ofthese elements may be owned and/or operated by an entity other than thecore network operator.

The MME 142 may be connected to each of the eNode-Bs 142 a, 142 b, 142 cin the RAN 104 via an S1 interface and may serve as a control node. Forexample, the MME 142 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 142 may also provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM or WCDMA.

The serving gateway 144 may be connected to each of the eNode Bs 140 a,140 b, 140 c in the RAN 104 via the S1 interface. The serving gateway144 may generally route and forward user data packets to/from the WTRUs102 a, 102 b, 102 c. The serving gateway 144 may also perform otherfunctions, such as anchoring user planes during inter-eNode B handovers,triggering paging when downlink data is available for the WTRUs 102 a,102 b, 102 c, managing and storing contexts of the WTRUs 102 a, 102 b,102 c, and the like.

The serving gateway 144 may also be connected to the PDN gateway 146,which may provide the WTRUs 102 a, 102 b, 102 c with access topacket-switched networks, such as the Internet 110, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and IP-enableddevices.

The core network 106 may facilitate communications with other networks.For example, the core network 106 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices. For example, the corenetwork 106 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 106 and the PSTN 108. In addition, the corenetwork 106 may provide the WTRUs 102 a, 102 b, 102 c with access to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

A problem that may occur in wireless communications systems supportingCS and PS sessions, is that a Service Request (SR) procedure mayestablish radio bearers when there is no pending user data, thusunnecessarily wasting radio resources. In the context of LTE systems,the Evolved Packet System (EPS) is a combination of the Evolved-UTRAN(E-UTRAN) access network and the Evolved Packet Core (EPC) network. Forexample, referring to FIG. 1C, the EPS is a combination of RAN 104 andCore Network 106.

The EPS may be an all IP type of network. The higher layer signalingbetween the WTRU and Core Network entity (i.e. the MME 142 in FIG. 1C)may be recognized as the Non Access Stratum (NAS) layer. At the NASlayer, the EPS may introduce two new protocol entities; the EPS MobilityManagement (EMM) as well as the EPS Session Management (ESM). Thesignaling connection between the WTRU and the MME is referred to as anEMM connection. The WTRU is in EMM-IDLE Mode if it is registered withthe network, but there is no EMM connection. Similarly, the WTRU is inEMM-Connected Mode when the (NAS) EMM connection exists. An EPS BearerContext is a collection of parameters and network resources that areallocated by the network upon a WTRU's request for various servicesrequiring different quality of service (QoS).

One purpose of the service request (SR) procedure is to transfer the EMMfrom EMM-IDLE to EMM-CONNECTED and establish the radio and S1 bearerswhen uplink user data or signaling is to be sent. Another purpose ofthis procedure is to invoke mobile originated (MO)/mobile terminated(MT) circuit switched (CS) fallback (CSFB) procedures. A SR procedure,regardless of its trigger (e.g., short message service (SMS) or userdata), may cause the establishment of radio and S1 bearers for allactive EPS bearer contexts in a wireless transmit/receive unit (WTRU).Thus, even if some triggering mechanism needs the signaling radiobearers, the mobile management entity (MME) may still establish theradio and S1 bearers for all active EPS bearer contexts if the WTRUsends an SR which is accepted by the MME.

Another way for the WTRU to request the establishment of radio and S1bearers is by setting a one-bit position (called the “Active Flag” bit)in the Tracking Area Update (TAU) request that is sent to the MME. Thismay happen if user data is pending when the TAU procedure is triggered.Setting this bit informs the MME that the non-access stratum (NAS)signaling connection may be maintained and the radio and S1 bearers maybe setup for all active EPS contexts.

When a WTRU sends a SR message, it starts timer T3417. This timer isthen stopped normally (i.e., the procedure completed normally) if theaccess stratum (AS) indicates the establishment of bearers for the userplane. However, a WTRU may need to establish a NAS connection forsending/receiving SMS, sending or receiving location serviceinformation, or other information. This is carried over signalling radiobearers and there may be no need to establish radio and S1 bearers forall EPS bearer contexts when the WTRU sent the SR for SMS or othernon-user plane related services. In this case, resources may be wastedand may be difficult to avoid.

According to a first embodiment, when a WTRU transitions from evolvedpacket system (EPS) connection management mode (EMM)-Idle mode toEMM-Connected mode, radio resources may be setup when the reason serviceis requested is for user data. This embodiment is described in furtherdetail below.

When the WTRU transitions from EMM-Idle mode to EMM-Connected mode, theradio and S1 bearers may be setup if the reason that service isrequested is related to user data, and may not be setup otherwise. Thus,if the WTRU is in Idle mode and is requesting non-EPS services (e.g.,SMS or supplementary services (SS)), location service, or positioningservices, the service request procedure does not cause the establishmentof radio and S1 bearers related to the active EPS bearers. One way toachieve this is to use a different establishment cause for the case whenuser data is pending or when the reason for the SR is not related topending user data, such as SMS, SS, location services, or other userdata.

Additionally, the WTRU may initiate the SR procedure in EMM-Connectedmode, to indicate the need for radio and S1 bearers for all active EPSbearer contexts. The WTRU may send an SR message or an Extended SR (ESR)message indicating a new type, for example “user data pending.” Thenetwork then may initiate the setup of radio and S1 bearers for allactive EPS bearer contexts. Because the SR message may not have aservice type information element (IE), a new IE may be included in theSR (or ESR) message to indicate the pending user data status which maythen trigger the establishment of radio and S1 bearers for the WTRU'scontexts. The IE may be a conditional or optional IE since the currentSR is four octets long. Thus, if the SR message is to be used asindicated above, this new IE may be included. Otherwise, the SR may notinclude this IE, (e.g., if it was sent to transition from Idle toConnected mode). This embodiment also applies to the ESR message.

Alternatively, the WTRU may send a new NAS message in Connected mode toindicate that the network must setup the radio and S1 bearers for allactive EPS bearer contexts.

For any of the above options (SR, ESR, or new NAS message), the WTRU mayeither specify the EPS bearer context for which it needs radio and S1bearers be set up, or the network may establish radio and S1 bearers forall active EPS bearer contexts. The former may re-use the EPS bearercontext status IE (or a new IE) to indicate what EPS bearer contexts areconcerned by the procedure.

Another problem that may occur in wireless communications systemssupporting CS and PS sessions, is that there may be a lack of means forinforming a WTRU to maintain a NAS signaling connection during aTracking Area Update (TAU) procedure.

When a WTRU is powered on, it may register in the EPS domain byperforming an “Attach” procedure. After the WTRU has attached to theEPS, it may have to inform the MME about its position using a TAUprocedure. A WTRU performing a TAU procedure may indicate to the networkthat there is more signaling to be done and/or there is user datapending by setting a bit, called the “Active Flag” bit, in the TAURequest message. If the bit is set by the WTRU, the network may thenestablish radio and S1 bearers for all active EPS bearer contexts. Butthere is no current mechanism for the network to indicate to the WTRUthat there is more mobile terminating (MT) signaling to be done even ifthe bit is not set in the TAU Request. Thus, it is possible that the MMEreceives SMS or other signaling that it needs to send to the WTRUwithout being able to do so.

In one embodiment, the network indicates to a WTRU to maintain NASsignaling during tracking area update (TAU) Accept signaling. A TAUresponse may be sent to the WTRU from the network with the updateresult. FIG. 2 is a block diagram of a tracking area update (TAU)response message 200 including an Evolved Packet System (EPS) Updateresult Information Element (IE) 210. TAU response message 200 includes afirst octet 208 and a second octet 210, where the second octet 210 isthe EPS Update result IE 210. The EPS Update result IE 210 includes anIE identifier 202, a spare bit 204, and an EPS update result value 206.The EPS Update result value 206 may inform the WTRU whether its updatingattempt has been successful for Tracking Area only or for both TrackingArea and Location Area. The fields of message 200 are shown with bitposition labels, for example the spare bit 204 is in bit position 4,however any arrangement of the fields may be used.

The spare bit 204 may be set to “zero”. The network may indicate to theWTRU that the NAS connection may be maintained for more pendingsignaling. For example, when a TAU request is sent to the MME, an MT SMSmay also arrive at the MME. Thus, to keep the NAS signaling connectionfor the SMS transfer, the MME sends a TAU Accept and sets the spare bit204 to value “one”. Upon reception, the WTRU maintains its NAS signalingand RRC connection with the MME and eNB, respectively, for a configuredtime during which the network may continue with more signaling. The WTRUmay release the connection if the time elapses without receivingsignaling from the network. The network may use the maintained NASsignaling connection to send SMS, SS, location, or positioning services,or to trigger ESM procedures, (for example, activation of a dedicatedbearer context which may be followed by an RRC message to establishradio and S1 bearers for all active EPS bearer contexts). The MME mayalso use the maintained signaling to inform the WTRU about a MT CSFBrequest.

Alternatively, the WTRU may expect some MT signaling before receivingthe RRC Connection Release message from the eNB. If this is the case,then the WTRU may process the signaling accordingly and may not considerthis as an error case.

Another problem that may occur in wireless communications systemssupporting CS and PS sessions, is that EPS bearer contextsynchronization may be limited within the TAU procedure. An EPS BearerContext is a collection of parameters and network resources that areallocated by the network upon a WTRU's request for various servicesrequiring different qualities of service. Every EPS bearer context thatis active in one entity (WTRU or MME/network) may have an activecounterpart in the other entity (MME/network or WTRU). Thus, the WTRUand the network try to maintain their EPS bearer contexts assynchronized as possible. In some cases, in the MME, for example, an EPSbearer context may be locally deactivated because it has not been in usefor some time. The MME may update the WTRU with the latest EPS bearercontext status by including an information element (IE) called the EPSbearer context status IE in certain signaling messages, such as the TAUAccept message.

Upon receipt of this IE by the WTRU, it may deactivate all those EPSbearer contexts locally (without peer-to-peer signaling between the WTRUand the MME) which are active in the WTRU, but are indicated by the MMEas being inactive. The same applies to the case if the WTRU has includedthe IE in certain messages such as TAU Request. If so, then the MME maydeactivate all those EPS bearer contexts locally (without peer-to-peersignaling between the MME and the WTRU) which are active on the networkside, but are indicated by the WTRU as being inactive.

The WTRU may perform a periodic TAU when its periodic timer expires. Ifthe WTRU has deactivated a context before the next TAU takes place, thenit may have to wait for the TAU initiation before synchronization withthe MME/network. It may also be possible that other requests arrivebefore the initiation of the periodic TAU; (e.g., CSFB for which it isimportant to take the latest EPS bearer context status into use as theprocedure leads to the reservation of corresponding resources in thetarget RAT). Thus, the synchronization process is not efficient becauseit is currently limited to TAU signaling.

In one embodiment, the EPS bearer context status IE may be included inany NAS messages. For example, the EPS bearer context status IE may beincluded in any or all NAS messages, (except where it may not bepossible, for example in a Service Request message), even if theprocedure is not the TAU procedure. Thus, every time there is an updateof the context's status, the WTRU/MME/network may include the bearercontext status IE in the next NAS message that is sent to the receiver,in order to promptly provide the latest context status.

According to an embodiment, the WTRU may include the IE in the ExtendedService Request (ESR) message. If the WTRU is in Connected mode when itsends the ESR message and it includes the EPS bearer context status IE,then the MME when executing the handover to a CS RAT may take theincluded status of the context into account when, for example,performing packet switched (PS) handover (HO) or establishing resourcesin the target RAT.

The MME/network may include the EPS bearer context status IE in allmessages, even the reject messages. For example, a Service Reject mayhave the EPS bearer context status IE included and the WTRU may updateits contexts accordingly when it receives this IE in such a message.This applies to both the connected mode and/or the Idle mode, and to theWTRU or the network.

Upon the receipt of the bearer context status IE, the receiver entity(WTRU or MME/network) deactivates all the EPS bearer contexts locally(without peer-to-peer signaling between the WTRU and the MME) which areactive in the receiver entity, but are indicated by the transmitterentity as being inactive.

FIG. 3 is a flow diagram 300 of a method for including the EPS bearercontext status in any NAS message. Steps 305-315 occur in a transmitterentity, and steps 320-325 occur in a receiver entity, where thetransmitter entity and receiver entity may be on the WTRU side and thenetwork side, respectively, or vice versa. In step 305, the transmitterentity detects an update in a status of an EPS bearer context in thetransmitter entity. In step 310, the transmitter entity includes an EPSbearer context status information element (IE) in a next Non-AccessStratum (NAS) message to the receiver entity reflecting the bearercontext status update. The next NAS message may be, for example, an ESRmessage. In step 315, the transmitter entity sends the next NAS messageto the receiver entity. In step 320, the receiver entity receives thenext NAS message. In step 325, the receiver entity deactivates all theEPS bearer contexts locally which are active in the receiver entity, butare indicated by the transmitter entity in the EPS bearer context statusIE as being inactive.

Other issues may arise in wireless communications systems supporting CSand PS sessions when the CS domain is temporarily unavailable. In thecontext of LTE systems, the concept of CSFB was introduced to allow aWTRU to get CS (or non-EPS) service over E-UTRAN, (i.e., MO/MT CS callsand SMS). The mobile switching center (MSC)/visitor location register(VLR) is a combination of a switch and a database that was introduced inthe 2G and 3G systems in order to provide the Circuit Switched (CS)services, and is located in the same operator's network as the MME. CSFBwas enabled by the introduction of a new interface between the MME andthe MSC/VLR known as the SGs interface.

For a WTRU to get non-EPS services, (e.g., SMS), a registration may beperformed on both the E-UTRAN and CS domains known as a combined attachprocedure. This combined registration creates an SGs association for theWTRU at the MME and the MSC/VLR. As such, an SMS message may be sentwith LTE signaling to the MME using native LTE NAS messages. If the WTRUhas an SGs association at the MME, the latter may then forward themessage to the MSC/VLR without its verification. Similarly, for MT SMS,the MME may forward the message to the WTRU after receiving the requestfrom the MSC/VLR via the SGs interface.

Similarly, if there is an MT CS call for a WTRU in E-UTRAN, the MSC/VLRmay create a paging message towards the MME using the SGs interface. Thelatter may then attempt to notify the WTRU about the MT call. The WTRUmay maintain an update status related to its EPS and non-EPSregistration outcomes, as follows.

The WTRU may maintain an EPS status of its current registration with thenetwork, where the EPS status may be any of the following: EU1: UPDATED,meaning the last Attach or TAU was successful; EU2: NOT UPDATED, meaningthe last Attach, Service Request (SR), or TAU attempt failedprocedurally, i.e., no response or a rejection was received from theMME; or EU3: ROAMING NOT ALLOWED, meaning the last Attach, SR, or TAUwas correctly performed but the reply from MME was negative (because ofroaming or subscription restrictions).

The WTRU may maintain a mobility management (MM) status of its currentregistration with the network (3G), where the MM status may be any ofthe following: U1: UPDATED, meaning the last location area update (LAU)was successful; U2: NOT UPDATED, meaning the last LAU failedprocedurally (no significant answer was received from the network,including the cases of failures or congestion inside the network); U3:ROAMING NOT ALLOWED, meaning the last LAU ran correctly but the answerfrom the network was negative; or U4: UPDATING DISABLED, meaning the LAUhas been disabled.

For the MM status, the subscriber identity module (SIM)/universalsubscriber identity module (USIM) may not contain any valid locationarea identity (LAI), temporary mobile subscriber identity (TMSI), globalsystem for mobile communications (GSM) ciphering key, universal mobiletelecommunications system (UMTS) integrity key, UMTS ciphering key, orciphering key sequence number. For compatibility reasons, all thesefields may be set to the “deleted” value at the moment the status is setto eCALL INACTIVE. However, the presence of other values may not beconsidered an error by the mobile station. Furthermore, if the mobileequipment (ME) supports any A5 ciphering algorithm that requires a128-bit ciphering key and a USIM is in use, then the ME may delete theGSM Kc₁₂₈ stored at the moment the status is set to eCALL INACTIVE. The“location update status” stored on the SIM/USIM may be “not updated.”

Thus, a WTRU may maintain both EPS and non-EPS statuses whenever it isperforming combined registration. In addition, whenever a WTRU performscombined registration (combined Attach or combined TAU), the VLR mayprovide a new TMSI and location area identity (LAI) to the WTRU whichare included in the TAU Accept message. The WTRU uses the TMSI when itgoes to the target radio access technology (RAT) if the list of LAI,provided to it did not change. A location update is triggered betweenthe MME and the VLR when the MME receives certain messages from theWTRU, (e.g., a combined TAU). But the periodic TAU which is governed bytimer T3412 does not cause a location update over the SGs interface.

It may be possible that the CS domain becomes temporarily unavailable,for example, if the MME resets and thus loses all the SGs associationsit had for the combined-registered WTRU. The same may occur if the VLRresets and then loses its SGs associations for the WTRUs that werepreviously combined-registered.

If the WTRU receives a Service Reject message (for example, with cause“#39—CS domain temporarily not available”), then the following actionsmay be taken. The WTRU may start a timer (for example time T3442) andset the EPS update status to “EU2 NOT UPDATED”, and store the status.Then, the WTRU may enter the “EMM-REGISTERED. NORMAL-SERVICE state”. TheWTRU may not try to send an extended service request message for mobileoriginating services to the network until the timer T3442 expires or theWTRU sends a TAU request message. Timer T3442, which is included in theService Reject message, may have a value that ranges from seconds, tominutes, or hours, or it may be deactivated.

With the existing WTRU behavior after receiving reject cause #39, asdescribed hereinbefore, it is possible that the WTRU does not receivenon-EPS services because of delayed combined registration. The followingproblems may be identified. If cause #39 is due to a VLR reset, thenthere may be no need to change the EPS update status as the MME stillhas the latest WTRU profile and context. Moreover, the MME may initiatea location update towards the VLR when the WTRU sends a periodic TAUmessage. In this case, the VLR may in turn provide a TMSI and locationarea identities in response, which are normally included in the TAUAccept message. But the WTRU may not expect these IEs in the TAU Acceptmessage when it is performing a periodic TAU message. Thus, the WTRUbehavior is unknown when it receives these IEs during a periodic TAUprocedure. As a result, the combined registration may be delayed, andthere may be delays associated with the non-EPS services. This delay isexplained further by FIG. 4A.

FIG. 4A is a flow diagram for a procedure 400A between a WTRU and anMME, including “Attach”, “Service Request” and “Tracking Area Update(TAU)” procedures, where there is an SGs Interface between the MME andan MSC/VLR. Steps 412, 430, 435, 440, 450 and 460 occur in the WTRU, andstep 415 occurs in the MSC/VLR.

As shown in FIG. 4A, in step 405, the WTRU may send a combined Attachmessage to the MME to register for both EPS and non-EPS services. Instep 410, the MME may respond with an Attach Accept message, causing theWTRU's EPS Update (EU) status and MM status to be set to EU1 (UPDATED)and U1 (UPDATED), respectively, in step 412. The WTRU may have beenassigned a TMSI and LAI IE as part of the Attach Accept message that itmay use for CSFB, supplementary services (SS), and LCS services.

In step 415, the VLR may restart and then inform the MME about the lossof all SGs associations for all the WTRUs. A MO CSFB request in the WTRUmay trigger an extended service request (ESR) message towards the MME,420, and the MME may reply with a Service Reject message, 425, due totemporary unavailability of the CS domain. The timer T3442 set to“deactivate” or N minutes, may also be included in the Service Rejectmessage. In step 430, the WTRU may change its EU status to NOT UPDATED,it may not modify the MM status, (which is set to UPDATED despite thecontrary), and it may start timer T3442 as indicated in the ServiceReject message. The timer may also have a value “deactivate”, whichimplies that the timer may not be started.

In step 435, when the WTRU goes to EMM-Idle mode, the WTRU may start itsperiodic TAU timer (T3412) running. In step 440, the timer T3412 mayexpire before timer T3442. If that is the case, then its expiry maytrigger the periodic TAU procedure by the WTRU sending a TAU message andsetting the update type to periodic update, in step 445. After sendingthe TAU message, in step 450, the WTRU may stop timer T3442 (amongstother timers) if it is running. In step 455, the MME may respond with aTAU Accept message, and in step 460 the WTRU may set its EU status toUPDATED.

If the MME does not autonomously perform a location update towards theVLR when it receives the TAU with update type set to periodic, theWTRU's combined registration may not be performed and the WTRU may notknow that there is a problem with the SGs interface, since the MM statusis U1 (UPDATED). There is no existing standard procedure and no actionis specified to be taken by the WTRU when T3442 expires.

If the CS domain is not available, other procedures may also beaffected, (e.g., the MO SMS procedure which is performed by sending theuplink (UL) TRANSPORT NAS message). But in the current WTRU behaviorwhen receiving cause #39, it is specified that the WTRU may not attemptan ESR until the combined registration is performed. But this does notstop other procedures, (e.g., MO SMS) which may not succeed.

In addition, if the SGs interface is down and the WTRU attempts to sendan SMS message with the UL NAS TRANSPORT message, there is currently nomessage back from the network to indicate the success or failure of theprocedure. This is because the behavior of the MME is such that itremoves the NAS header and forwards the encapsulated SMS message to theVLR. Any retransmission and errors are handled by the SMS layer. Assuch, if the WTRU sends an SMS message which does not arrive at itsdestination due to the SGs being faulty, the WTRU's SMS entity mayretransmit without success since a combined registration may be neededto solve the problem.

The following embodiments define new WTRU and MME behavior to handle thevarious situations that arise when the SGs Interface, or CS domain, isunavailable.

One embodiment is illustrated in FIG. 4B. FIG. 4B is a flow diagram fora procedure 400B performed by a WTRU when if is informed that the CSdomain is unavailable. In step 470, the WTRU is informed that the CSdomain is either unavailable or temporarily unavailable. For example,when the WTRU tries to contact the MME for service, it may receive aService Reject message with a cause value equal to “CS Domain notavailable” or “CS domain temporarily not available”.

In step 472, the WTRU determines if the problem with the SGs Interfaceis due to the MSC/VLR and not the MME. This determination is based onthe notification from the MME. If the cause value is “CS Domain notavailable”, then that means there is a problem with the SGs interfaceand the MME can't connect. If the cause value is “CS Domain temporarilynot available”, then the SGs Interface is fine but there may be aproblem with the MSC/VLR.

If the problem is not due to the MSC/VLR, then in step 474 the WTRUchanges its mobility management (MM) update status to indicate that theregistration to CS domain was unsuccessful. For example, the WTRU maychange its MM update status to U2: NOT UPDATED.

The WTRU's EU status may be unchanged (i.e., it remains set to EU1:UPDATED) if the problem with the SGs is due to the MSC/VLR and not theMME, proceeding to step 476. In step 476, upon the next occurrence of anevent that triggers a TAU, or for every TAU procedure, the WTRU maycheck its MM update status. In step 478, if the WTRU is registered incombination, (or not, depending if the previous TAI did not have non-EPSservices), then the WTRU may perform a combined registration, (e.g.,combined TAU), procedure. The combined registration procedure involvessetting the update type to “combined TA/LA updating with internationalmobile subscriber identity (IMSI) attach” or “combined TA/LA updating”,depending on the scenario.

According to another embodiment, if the unavailability of the CS domainis due to problems in the MSC/VLR, then the WTRU may consider anyallocated temporary mobile subscriber identity (TMSI) and LAI asinvalid.

According to another embodiment, if the Service Reject is due to theWTRU sending an ESR for an emergency call, then the WTRU may change itsRAT to a CS RAT where it may place an emergency call. The WTRU mayreselect to a CS RAT if a mobile originated (MO) CS call is requestedthat is not for emergency purposes. This may always be done regardlessof the WTRU's mode of operation, or it may be done if the WTRU isoperating in a mode for which the CS domain is preferred or the WTRU isvoice centric, and there is no support for internet protocol (IP)multimedia subsystem (IMS) voice calls (or other forms of voice overIP). The unavailability of IMS voice calls may also mean unsuccessfulregistration to the IMS network.

According to another embodiment, if the WTRU receives a reject messagewith cause #18—“CS domain not available”, then the WTRU may perform acombined TAU whenever a TAU procedure is initiated for any reason. TheWTRU's behavior, (i.e., with regard to performing combinedregistrations), may return to the expected behavior upon the firstsuccessful combined registration procedure.

According to another embodiment, the WTRU may perform a combinedregistration upon the expiration of timer T3442 (i.e., expiration ofthis timer will trigger a combined registration). This may occur if theWTRU is in connected or idle mode.

According to another embodiment, when the WTRU receives cause #39, itmay not send ESR until timer T3442 expires or a TAU Request is sent. TheWTRU may not send the ESR before a combined TAU. Thus, the WTRU maytrigger the combined TAU upon expiry of T3442 and then decide if ESRand/or other non-EPS services may be requested based on the result ofthe update. Moreover, if the timer T3442 is deactivated, (or has a valueset to ‘deactivated’), then the next TAU may be a combined TAU. Based onthe results, the WTRU may decide if ESR or other procedures related tonon-EPS services may be initiated.

According to another embodiment, the EMM may inform the SMS entity,(and/or other entities that are affected by the unavailability of the CSdomain), about the unavailability of the CS domain. Thus, the affectedentities may not initiate any procedure that requires the SGs interfaceuntil a combined registration is successfully performed. Moreover, theEMM entity may not initiate the UL NAS TRANSPORT procedure when it knowsabout the unavailability of the CS domain. For example, if the WTRUwants to send an SMS message when it is in Idle mode, the WTRU firstsends a Service Request (SR) procedure and then sends the UL NASTRANSPORT which encapsulates the SMS message. Thus, the SR procedure maynot be initiated due to SMS if the EMM, (and/or SMS and other entities),know about the unavailability of the CS domain.

This embodiment may also apply to other services, (e.g., locationservices). That is, the EMM may inform all affected entities in the WTRUabout the unavailability of an interface between the MME and a peerentity in the network. No NAS procedures may be triggered to access thatservice until the interface in question is restored. For example, theWTRU may not send a SR message to transport information related tolocation services via an UPLINK RELIABLE TRANSPORT message and an UPLINKRELIABLE TRANSPORT RESPONSE message which are used for locationservices. Moreover, procedures such as CSFB procedures may have a higherpriority over any ongoing procedure related to location services. Thus,the WTRU may choose to ignore an ongoing transport of location serviceinformation and continue with a pending CSFB request.

According to another embodiment, the EMM entity may inform the MMentity, (and/or other entities that are affected by the unavailabilityof the CS domain, e.g., SMS, etc.), that the CS domain is now available.At this point, the WTRU may start sending ESR or may initiate the UL NASTRANPSORT procedure.

According to another embodiment, if the MME receives a reset indicationfrom the MSC/VLR, then the MME may initiate a location update on the SGsinterface when it receives any type of TAU, (i.e., regardless of theupdate type that is set by the WTRU), from a WTRU which was previouslycombined registered. In addition, if the location update over the SGs issuccessful, the MME may include any allocated TMSI and/or LAI in the TAUAccept message regardless of the update result type set by the MME. TheMME may also set the update result type to something different from whatis expected by the WTRU. For example, if the WTRU successfully performsa periodic TAU, it expects a TAU Accept from the MME with the updateresult set to “TA updated.”

The MME may set the update result to “combined TA/LA updated” if the MMEinitiated the location update towards the MSC/VLR on the SGs interface.When the WTRU receives the TAU Accept, it may use either the updateresult type to learn about the status of its combined registration, orit may conclude about its combined registration based on the presence(or absence) of an LAI IE.

If the update result is set to “combined TA/LA” then the WTRU mayconclude that it is successfully registered for both EPS and non-EPSservices. The WTRU may change its MM status to U1 updated and may handlethe allocated LAI and possibly the TMSI as previously described when theWTRU's combined registration is successful with these IEs included. If aTMSI is not included in the TAU Accept, the WTRU's behavior may be thesame as previously specified when the registration is successful withouta new TMSI being included.

According to another embodiment, if the update result is “TA updated”and the TAU Accept contains an LAI, then the WTRU may consider itscombined registration as successful. The WTRU may change its MM statusto U1 updated and may handle the allocated LAI and possibly the TMSI aspreviously described when the WTRU's combined registration is successfulwith these IEs included. If a TMSI is not included in the TAU Accept,the WTRU's behavior may be the same as previously specified when theregistration is successful without a new TMSI being included.

According to another embodiment, more details may be provided in thereject causes to indicate the source of the problem for theunavailability of the CS domain, (i.e., whether it is the MME that hasrestarted or the MSC/VLR). A new reject cause may be introduced toexplicitly indicate the cause of the problem so the WTRU may takespecific actions. For example, if the MME is the entity that hasrestarted, then providing this information to the WTRU may cause achange to the EU update status and not the MM status. Moreover, the WTRUmay later perform a combined registration and may set the update type to“TA/LA updating” since it is already registered to the CS domain.

According to another embodiment, the value of the timer T3442 may beused to indicate the source of the problem. For example, setting thevalue of the timer to “deactivated” may be used to inform the WTRU thatthe MME is the source of the problem, and the WTRU may perform acombined registration. Other values for the timer may be used to signalthis scenario or other scenarios. For example, other values may be usedto indicate that the MSC/VLR is the cause for the unavailability of theCS domain. The WTRU may then behave as explained earlier.

According to another embodiment, a new message may be defined to informthe WTRU about the unavailability of a certain domain or interface whenthe procedure is not CSFB. For example, the WTRU may send an UL NASTRANSPORT message for MO SMS. If the CS domain is not available (or theSGs interface is not functional due to errors at either MME or MSC/VLR)then the WTRU may be informed about the problem using the new message.

A new generic message may be defined to signal such problems for allinterfaces depending on the values of certain IEs. For example, a newNAS message called SERVICE UNAVAILABLE NOTIFICATION may be defined toindicate to the WTRU the unavailability of a domain or interface. An IE,(e.g., Notification Type), may be included in this message and dependingon its value, the WTRU may be notified about unavailability of the CSdomain, (or the SGs interface), or the unavailability of the interfacebetween the MME and a network node such as that for location services.The MME may send this message when it receives a NAS message from theWTRU that requires forwarding over a certain interface. Upon receptionby the WTRU, the triggering entity may be notified about the error andthe procedure may not be re-attempted until the interface is restored,(e.g., by some sort of registration or a notice from the network).Moreover, a timer may be included to indicate when the WTRU mayre-attempt the unsuccessful procedure or re-attempt a registration whichif successful may allow initiation of the procedure.

Another way of achieving this procedure is to use an existing NASmessage to indicate the problem. For example, the EMM INFORMATION may beused to achieve this. The EMM entity may also inform all other affectedentities when a particular message indicating an error in an interfaceis received. For example, if the WTRU is informed about unavailabilityof the CS domain when the MME receives UL NAS TRANSPORT, then the EMMmay inform the MM entity so that ESR may not be initiated until asuccessful combined registration is performed.

In any of the above embodiments, the combined registration may take anyof the conventionally specified types of combined registrations. Thefollowing are provided as examples: combined attach with attach type setto “EPS/IMSI attach; combined TAU with update type set to “combinedTA/LA updating; or combined TAU with update set to “combined TA/LAupdating with IMSI attach.

Moreover, the above embodiments also apply to WTRUs that are registeringfor the first time for both EPS and non-EPS services. Alternatively, inthis case, the regular behavior for unsuccessful registration fornon-EPS services only may be followed by the WTRU, (e.g., theregistration attempt counter or a similar counter is incremented up to amaximum value after which other actions may be taken by the WTRU). Inaddition, there may be a time limit between each attempt. This may alsobe applied to WTRUs that have been previously successfully registeredfor both EPS and non-EPS services and later receive a reject messagewith cause #39, (CS domain temporarily not available), or #16, (MSCtemporarily not reachable), or #18, (CS domain not available).

All of the above embodiments and examples apply to the case when the CSRAT is both 3GPP or non-3GPP, and the same embodiments and examplesapply for non-EPS services, (CSFB, SMS, SS, etc.), to the case whenthere is a different interface between the MME and a non-3GPP network,e.g., CDMA2000 (1xRTT). The above examples may be used in anycombination.

A problem that may occur in wireless communications systems supportingCS and PS sessions, is that significant delays may occur when performingCSFB for emergency calls.

FIG. 5A is a flow diagram of a procedure 500A in response to a WTRUplacing an emergency call. Some steps of the procedure are omitted. Ifthe WTRU is attached for both EPS and non-EPS services and wants toplace an emergency call, in step 505 the WTRU may send an ESR message.The WTRU may set the service type to “mobile originating CS fallbackemergency call” or “1xCS fallback emergency call” depending on the CSdomain type supported by the WTRU. If PS HO is supported in the targetRAT, then in step 510 the WTRU performs a PS HO before continuing withplacing the CS emergency call. In step 515, the CS call establishmentprocedure is initiated in order to place the CS emergency call. As canbe seen from FIG. 5A, the CS call can only be initiated after the PS HOcompletes. However, since the call requested is an emergency call, it isimportant to reduce any delays to a minimal level.

Another scenario in which CSFB may be delayed occurs when a WTRUrequests a service from a closed subscriber group (CSG) cell that it isnot allowed to be on. This can happen, for example, when the WTRU'ssubscription has expired but the WTRU is not yet updated with thesubscription information, or when a WTRU is on a hybrid CSG cell andpriority is given to subscribers.

In this case, the WTRU's ESR may be rejected with cause code #25 and theWTRU's behavior is such that it has to look for a suitable cell as itenters limited mode. Thus, the WTRU, after finding a suitable cell, willneed to re-start the procedure for CSFB again. Thus, this causes delaybecause the CSFB has not been processed yet. Reducing the delay isespecially important if the CSFB is triggered due to an emergency call.

If the WTRU knows, based on its fingerprint, that it is near a 3GHNB/CSG cell, then it will send a proximity indication to the currentserving eNB (E-UTRAN) which might configure the WTRU with measurementgaps in order to read the system information messages in UTRAN in orderto acquire the CSG identity. Since reading the system information blocks(SIBs) on UTRAN system will require some time, existing solutionsproposed to reduce the delay to a minimal level include: 1) The WTRUdoesn't send a proximity indication when it is performing CSFB requests,if it knows that there is at least one other macro cell available in thelegacy RAT where it can place its CS call; 2) the WTRU sends itsproximity indication and the network doesn't request the WTRU to readsystem information messages from any HNB, if the network knows aboutavailability about at least one macro cell of the legacy RAT; or 3) bothWTRU and the network consider any HNB in the target RAT as the leastpriority cell in order to avoid possible system information reading andaccess check.

According to an embodiment, the delays for CSFB Emergency calls may bereduced by not performing a PS HO and redirecting the WTRU to the targetsystem. In order to reduce the delays related to CSFB procedure foremergency calls, the LTE network may not perform PS HO, (even if thetarget network supports it), and redirect the WTRU to the target system.This may be achieved in many different ways, as described in thefollowing embodiments.

FIG. 5B shows a flow diagram of a procedure 500B for reducing delay inthe CSFB procedure for emergency calls. In step 530, the WTRU sends aESR for an emergency call. The ESR message may have the service type setto “mobile originating CS fallback emergency call” or “1xCS fallbackemergency call”. In response, the MME does not perform PS HO for theWTRU. Instead, the MME sends a WTRU Context Modification Request to theeNB in step 535 in order to redirect the WTRU directly to the CS domain.This is accomplished in step 540 by the eNB sending to the WTRU either anetwork assisted cell change order message or an RRC connection releasemessage, where the message includes the redirection information for theCS domain. The WTRU then attempts to access the CS domain and continuefirst with establishment of the CS call for emergency services.

The WTRU may indicate to the MME that PS HO is not preferred. This canbe done by including an EPS bearer context status IE in the ESR andsetting the values for all contexts to indicate that they have beendeactivated. Alternatively, the WTRU can include a new IE to indicatethat PS HO is not preferred.

Other scenarios in which delay can be seen include when a WTRU is on aCSG cell for which the subscription has expired. In this case, an ESRcan be rejected with cause #25 and the WTRU would need to find asuitable cell and then send the ESR again. In order to reduce the CSFBdelay in such a scenario, (for normal CS calls or for an emergency CScall request, which is identified by having the request type in the ESRset to mobile originating CS fallback emergency call or 1xCS fallbackemergency call), the following embodiment may be used as means to reducecall setup delay.

According to one embodiment, the MME may ignore the expiry of thesubscription for the exceptional case of CSFB requests and accept theESR. The WTRU can later be updated with the latest subscriptioninformation, for example, upon subsequent access retries on the CSGcell. Thus, it is proposed that the reject cause #25 may not be appliedto CSFB related procedure or signaling.

According to another embodiment, the WTRU may also be handed over toanother allowed/suitable LTE cell from which the CSFB procedure iscontinued. In this case, the ESR is not rejected and the WTRU does nothave to go through cell reselection, hence avoiding potential delays.The network then continues the ESR procedure from the target cell onwhich the WTRU has completed the intra-LTE handover.

According to another embodiment, the network may conditionally reject,(or accept), the ESR procedure. That is, the network may allow the ESRprocedure to go through while indicating to the WTRU that the CSGsubscription has expired. This can be achieved in RRC or NAS signalingor a combination. For example, the network can send the inter-systemchange message, (e.g. mobilityFromEUTRACommand), and include anindication of the WTRU's subscription, (e.g. expired). Thus the WTRU maycontinue the CSFB procedure but also use the provided indication toupdate the list of allowed CSGs. If provided within RRC messaging, theRRC layer can provide the NAS with such indications so that theappropriate update is performed on the allowed CSG list. This can bedone, for example, with one bit in which a specific value indicatesexpiration of a subscription for the CSG cell. Additionally, the MME canprovide such an indication over an interface, (such as the S1-APinterface), to the eNB which can use such indication to take certainactions such as including a one bit indication in the RRC messages.

The WTRU and the network may reduce any potential delays that arerelated to falling back to a target cell which is a CSG cell. Toaccomplish this, according to another embodiment, the WTRU mayprioritize GERAN since CSG is not supported there. As such, anypreliminary access check that is needed to be performed by the WTRU isautomatically avoided.

The WTRU may not send a proximity indication to the eNB to indicate thatit is close to a target CSG cell of UTRAN, or any other system. By doingso, the network will not schedule the WTRU for measurement and systeminformation reading on a target UTRAN CSG cell. Avoiding this willeliminate potential delays if a UTRAN CSG cell needs to be verified foraccess eligibility by the WTRU and the network. The WTRU may take suchan action if it knows that there are other neighboring inter-RAT macrocells. Alternatively, even if the WTRU sends a proximity indication toinform the network that it is near a CSG cell, it is proposed that thenetwork should not act upon this indication and therefore not schedulegaps for the WTRU to go and read system information message in order toperform preliminary CSG access check. Similarly, this can be done if thenetwork knows that there are other UTRAN macro cells around the WTRU.

According to another embodiment, if CSFB is performed with networkassisted cell change order or by releasing the RRC connection, (may bewith redirection information), and system information of at least onetarget cell is provided to the WTRU, then the following may beperformed.

In this case, the WTRU may prioritize cells that may not require achange in location area identity (LAI), (e.g. the WTRU may have one inLTE). This may be done by checking the LAI that the WTRU has against theLAI of each potential target cell which may be derived from the providedSIBs or system information. Moreover, the WTRU may give priority tomacro cells within the desired LAI in order to avoid possible accessreject due to CSG subscription invalidity. This may be done even if theWTRU has the associated CSG ID in its lists, and regardless of thesignal power and/or quality received from CSG cells or cells that fallout of the desired LAI. Thus, the cell selection rules may be overruledin this case. Alternatively, there may be a minimal threshold beyondwhich the WTRU may not apply, for example, preconfigured thresholds inthe WTRU or signaled using RRC or NAS messages. Cells that fall within adesired LAI, provided over LTE, may avoid delay due to possible locationarea update (LAU) requests.

If the WTRU receives signals from the macro cells within the desired LAIwhich are weak or fall below a certain threshold, the WTRU may givepriority to a CSG cell within the LAI. This prioritization is based onthe criteria of CSG permission (based CSG ID check, or otherwisesignaled to the WTRU during the inter-system change).

The last priority may be given to cells utilizing CSFB that belong to adifferent LAI in which macro cells may have higher priority than CSGcells. The above examples and embodiments for CSFB prioritizationdescribed hereinbefore may be applied in any combination.

FIG. 6 is a flow diagram of a procedure 600 when the WTRU performs aninter-system change when a location area has changed. In step 602, theMSC/VLR sends a Paging Request to the MME (e.g. SGsAP-Paging-Requestmessage). In step 604, the MME sends a Paging Request to the eNB and instep 606, the eNB sends a Paging message to the WTRU. In steps 608, theWTRU responds to the Paging message by sending a ESR message to the MME.In step 610, the MME sends a WTRU Context Modification Request (withCSFB indicator) to the WTRU. In step 612, the WTRU sends measurementreports to the eNB.

If the WTRU performs an inter-system change, possibly with PS HO, andfinds out that the location area has changed, the location area updateprocedure is performed in step 615. In step 620, the CS MO voice call isinitiated by the WTRU toward the MSC. As can be seen from FIG. 6, thelocation area update procedure is performed before a CS voice call canbe performed.

In order to avoid this problem, according to another embodiment, theWTRU may use, (in the target CS cell/RAT/system whenever performingCSFB), its IMSI in its initial message, (e.g. Paging response in MT caseor CM Service Request in MO case), for both mobile originating andmobile terminating cases. As such, even if the MSC/VLR has changed, theIMSI in the CM Service Request will uniquely identify the WTRU in thenew MSC/VLR and hence a location update is not necessary before the callis set up.

According to another embodiment, a TAU may be triggered when CSFB fails.FIG. 7 is a flow diagram of a procedure 700 for a TAU being triggered inresponse to a CSFB failure. In step 705, the WTRU detects that a CSFBprocedure failed. For example, the NAS may use indications from the RRCthat the CSFB procedure has failed to send the TAU request. In step 710,the WTRU sends a TAU request, possibly with the type being a combinedTAU, in response to the CSFB procedure failure. This procedure may allowthe WTRU to resume in LTE with all its contexts it previously had beforethe CSFB procedure initiation. In step 715, the MME receives the TAUrequest. In step 720, the MME uses the TAU request as an indication ofthe failure and may take additional actions such as re-initiatinganother CSFB request or informing other network nodes about the failuree.g. (the SGSN or MSC/VLR).

Another problem that may occur in wireless communications systemssupporting CS and PS sessions, is that delays may occur when performingCSFB for a CS call that terminates at another CFB capable WTRU.

It is possible that a WTRU that is combined attached, (for EPS andnon-EPS services), originates a CSFB procedure in order to place a CSvoice call whereby the called party is another CSFB capable WTRU that isalso in LTE coverage. Thus, for the voice data to be exchanged betweencalling WTRU, (known as WTRU 1), and the WTRU that is being called,(known as WTRU 2), there is need to perform two CSFB procedures, onemobile originating (MO) CSFB procedure that triggers a mobile terminated(MT) CSFB procedure.

The MO CSFB includes signaling associated with WTRU 1's request to placeCSFB, which includes some optional procedures, for example, a PShandover may occur if WTRU 1 and the network support PS handover. Also,if the MSC/VLR that receives the CM Service Request is different, thenthe CM Service Request by WTRU 1 may be rejected and the WTRU 1 mayperform a LAU before it can continue with the CS call. At this point,the WTRU 1 may send the Call Setup message to the MSC/VLR, whichcontains the address of the called party, WTRU 2.

The MSC/VLR verifies the called party address and sees that there is anSGs association for WTRU 2, (i.e. the MME has triggered non-PSregistration over the SGs interface due to the reception of a combinedregistration from WTRU 2 in LTE coverage). Accordingly, a MT CSFBprocedure is triggered towards WTRU 2. In the mean time, WTRU 1 iswaiting since WTRU 2 is still in LTE coverage. It only after WTRU 2completes the inter-system change that voice data can be exchange overthe CS domain.

The signaling described above creates delays associated with thisscenario and prolongs the waiting time for WTRU 1. This waitingtime/delay may be reduced if the CSFB procedure for WTRU 2 is initiatedearlier and with fewer unnecessary delays.

According to one embodiment, the delays for CSFB to CSFB calls arereduced by initiating the second CSFB call sooner. For a mobileoriginated (MO) CSFB request that triggers a mobile terminated (MT) CSFBprocedure, the delay may be reduced if the CSFB procedure for WTRU 2 isstarted earlier than usual, before WTRU 1 completes its CSFB procedure.To perform such a procedure, when WTRU 1 is performing an MO CSFBprocedure, (i.e. by sending an Extended Service Request (ESR) to theMME), the WTRU may include all the necessary information regarding thecalled party in the ESR. The MME, after receiving an ESR, may forwardthis addressing information to the MSC/VLR. The MSC/VLR checks theaddress of the called party to see if there is an SGs Interfaceassociation at the MSC/VLR, or if the called party is a fixed linenumber or an emergency call center.

If the called party is another WTRU with a valid SGs association, theMSC/VLR may start a MT CSFB procedure for this WTRU by sending thenecessary message, the SGsAP-Paging-Request, over the SGs interface. TheMSC/VLR may indicate to the MME that the call is triggered by anotherCSFB procedure. The MME may use this information to take any necessaryaction that can speed up the MT CSFB procedure. Moreover, the MME mayinform the WTRU that the MT CSFB is from another CSFB capable WTRU. TheWTRU may use this indication to speed up the procedure e.g. start takingmeasurements and sending reports. This functionality may be achieved byusing any of the following embodiments.

In another embodiment, WTRU 1 sending an ESR may include the necessaryinformation that may be used to identify the destination. For example,some or all of the Call Setup message can be included in the ESR in theform of a new IE or a new container. Alternatively, the WTRU 1 may notinclude this information if it knows that the destination is not CSFBcapable, (for example, the destination has a fixed line number or anemergency call center). In some countries mobile subscribers and fixedsubscribers have distinct numbers series that may be used to determinewhether the B number series refers to fixed or mobile subscriber.

In another embodiment, a new message (e.g. SGsAP-MO-CSFB-Indication) maybe needed to carry this information from the MME to the MSC/VLR over theSGs interface. The MME may remove all the addressing information fromthe ESR and may insert it in this new message that is to be sent overthe SGs to the MSC/VLR. Alternatively, the MME may send this message asan indication that a MO CSFB procedure has been triggered and theMSC/VLR is to expect a WTRU for a CS voice call. The MSC/VLR may usethis indication to take statistics, (e.g. number of call failure, setuptime, etc). The MME may start a timer upon the sending of this message.

In another embodiment, the MSC/VLR upon reception of this message, mayverify if the destination called party is another WTRU with SGsassociation (this might involve interaction with other nodes e.g. HLR).If not, the MSC/VLR may take no further actions. The MCS/VLR mayacknowledge the receipt of the indication from the MME by sendinganother message, (for example SGsAP-MO-CSFB-ACK). The MME may use thisacknowledgement to take any necessary action, (e.g. stop any timer thatit might have started). If the MSC/VLR has a SGs association for thecalled party, the MSC/VLR may trigger the SGsAP-Paging-Request towardsthe MME. The MSC/VLR may include an acknowledgement for the previousmessage sent by the MME, for example by including a one bit position inthe SGs-AP-Paging-Request message.

In another embodiment, the MME may then stop any timer based on theacknowledgement received. In addition, the MME may start the MT CSFBprocedure by requesting the E-UTRAN to page the WTRU, (if it is in idlemode), or by sending the CS Service Notification to the WTRU, (if it isin connected mode). In either case, the MME may indicate that this CSFBis triggered by another CSFB procedure.

In another embodiment, if the WTRU 1 has completed its CSFB procedureand is in the CS domain, the MSC/VLR may start sending some information,(e.g. ringing tone), to indicate that the call request is beingprocessed. The MSC/VLR may choose whether a ringing tone or any othercall in progress tone is used based on whether or not the WTRU 2 is aCSFB subscriber waiting for the CSFB procedure to be completed.

FIGS. 8A and 8B show example CSFB procedures for two CSFB calls withenhancements, 800A, and without enhancements, 800B, respectively. InFIG. 8A, WTRU 1 initiates a CSFB call back by sending an ESR for CSFB802, providing measurement reporting 804, initiating a PS HO 806,performing a location update 808, and setting up a CS call 810. TheMSC/VLR initiates paging over the SGs interface, 812. Then, the MME mayinform WTRU 2 about the MT CS voice call, 814. Subsequently, WTRU 2initiates its an inter-system change while WTRU 1 waits. WTRU 2 acceptsthe CSFB 816, provides measurement reporting 818, initiates a PS HO 820,performs a location update 822, and responds to paging 824.

According to an embodiment, the delays experienced by WTRU 1 while WTRU2 initiates an inter-system change may be reduced using the procedure inFIG. 8B. Referring to FIG. 8B, WTRU 1 initiates a CSFB call back bysending an ESR for CSFB and includes the address of the called party(WTRU 2) 826. The MME forwards addressing information to the MSC/VLR828, and the MSC/VLR initiates paging for WTRU 2 over the SGs interface,830. In 832, the MME informs the WTRU about the MT CS voice call. Inparallel, WTRU 1 provides measurement reporting 804, initiates a PS HO806, performs a location update 808, and sets up a CS call 810. WTRU 2accepts the CSFB 816, provides measurement reporting 818, initiates a PSHO 820, performs a location update 822, and responds to paging 824. Incontrast to the scenario in FIG. 8A, the procedure in FIG. 8B reducesthe waiting time for WTRU 1 for the CS voice exchange.

Another problem that may occur in wireless communications systemssupporting CS and PS sessions, is that a WTRU may be made to fallback toCS when the MT call is cancelled. It may be possible that a mobileterminating (MT) CSFB request is cancelled by the caller. In this case,the MSC/VLR receives a request for a CS call from WTRU 1, the MSC/VLRinitiates an MT CSFB procedure towards WTRU 2, and the MSC/VLR thenreceives a request by WTRU 1 to abort the CS voice call request. Thus,if the MT CSFB procedure continues, WTRU 2 may perform an inter-systemchange to GERAN/UTRAN, possibly with PS HO, but may not receive any CScall as it may have been aborted by WTRU 1. This inter-system change isunnecessary and should be avoided. It may cause degradation of serviceif PS HO is not supported, or if it is supported but the QoS provided bythe target system does not match the source system. Recall that theMSC/VLR may start an MT CSFB procedure for a WTRU by sending theSGsAP-Paging-Request the over the SGs interface to the MME.

According to one embodiment, inter-system change is avoided for a WTRUwhen a CS paging or call request that triggered a MT CSFB is cancelled.If a CS call request that triggers a MT CSFB call is cancelled, thecalled WTRU (which is CSFB capable and assumed to possibly be in LTE)may not be made to perform an inter-system change even if it has beennotified of the MT CSFB procedure. In order to do so, the procedureshown in FIG. 9 may be used.

FIG. 9 is a flow diagram of a procedure 900 for avoiding an inter-systemchange for a WTRU when a CS call request that triggered a MT CSFB iscancelled. In step 905, the MSC/VLR decides to abort a MT CSFB call thathas been placed to a WTRU. The MSC/VLR may make the decision to abortthe MT CSFB call because it receives a request from the WTRU to abortthe MT CSFB call. Alternatively, the MSC/VLR may have started a timerwhen it sent a paging message to the WTRU to establish the MT CSFB call,and may decide to abort the MT CSFB call when the timer times out beforereceiving a response from the WTRU.

In step 908, the MSC/VLR determines if a paging procedure to start theCSFB procedure has been triggered. For example, it determines if aSGsAP-Paging-Request has been sent. If the paging request has not beensent, then in step 915, the MSC/VLR aborts the paging request(SGsAP-Paging-Request) procedure. Otherwise, if the paging procedure hasbeen triggered, in step 910, the MSC/VLR sends an abort message, (e.g.SGsAP-MT-CSFB-Cancel) to the MME to request the MME to the cancel the MTCSFB call. The MSC/VLR may start a timer after sending this message.

In step 920, the MME receives the cancellation message from the MSC/VLR.Subsequently, the MME may take certain actions to abort the inter-systemchange for the WTRU under consideration. In step 925, if the WTRU hasalready sent the ESR (i.e. the MT CSFB has been accepted) but the MMEhas not executed the inter-system change yet, the MME may send a ServiceReject with a new cause, for example, indicating “CS Call Aborted”.

Additionally, the MME may send an abort request to other network nodessuch as the SGSN that are possibly part of the procedure. In addition,the MME may send an acknowledgement to the MSC/VLR to acknowledge thereception of the abort message (e.g. SGsAP-MT-CSFB-Cancel) and may alsoinclude a status IE to indicate the result of the request (e.g. “MT CSFBCancel Success” or “MT CSFB Cancel Failure”).

In step 930, if the WTRU has been made to perform the inter-systemchange, the MME may not send any further messages to the MSC or theWTRU. By taking no further action, the MME permits the WTRU to fall backto the other RAT. Alternatively, the MME may send an acknowledgement tothe MSC/VLR and indicate that the cancel request is not successful. TheMSC/VLR may use this indication to take further actions, for example, tostop any timer that has been started.

Upon reception of the Service Reject with cause “CS Call Aborted”, theWTRU may take any necessary action, (e.g. the EMM entity may inform theMM entity about the abortion of the procedure). The WTRU may resumeoperation in LTE.

If the MME had not yet informed the WTRU about the MT CS voice request,then upon the reception of the cancel indication from the MSC/VLR, theMME may ignore the initial request and not inform the WTRU about the MTCS call. Alternatively, if the MME has requested E-UTRAN to page theWTRU for CSFB request, the MME may request E-UTRAN to abort the pagingprocedure. The E-UTRAN may acknowledge the request and inform the MMEabout a successful paging as described above.

A problem that may occur in wireless communications systems supportingCS and PS sessions, is that there may be undefined behavior for MT CSFBin Connected Mode.

A WTRU in Connected mode may be notified about a MT CSFB request usingthe CS Service Notification that is sent by the MME. The WTRU mayrequest upper layers' input to accept or reject CSFB before respondingwith an ESR message. The response may be indicated in the CSFB responseIE in the ESR message in both EMM-IDLE and EMM-CONNECTED modes. Theproblem identified with this is that it may be possible that a WTRU isin connected mode actively downloading a file but the user is notnecessarily next to the WTRU. Thus, if the WTRU receives a CS ServiceNotification message and requests input from upper layers to accept orreject a call, such input might not be provided if the user is away fromthe WTRU. Therefore, the expected behavior is undefined (for example, itis unknown if an ESR message will be sent to the network/MME and if sothen the CSFB response information element). Moreover, it is notefficient that the WTRU takes a decision on behalf of the user andaccepts the call since PS HO might not be supported in the target RATand service disruption may occur. At the same time, not providing anyfeedback to the MME may prevent other actions from being taken, such asthe VLR may forward the call to the voice messaging centre. Thefollowing embodiment may be used independently or in any combination toaddress this problem.

According to another embodiment, the WTRU and MME behavior is definedfor MT CSFB in connected mode when user input is requested but without auser response. The MME may start a timer after, or at the time, it sendsthe CS Service Notification message. The timer may be set to any valuethat is deemed appropriate for the scenario. For example, the value maybe set to 5 seconds. If the timer expires before receiving any responsefrom the WTRU (i.e. ESR), the MME may send a message (e.g. over the SGs)to the MSC/VLR to indicate that no response was received from the WTRUeven though the MME is in contact with the WTRU. A new message may bedefined or an existing message of the SGs protocol may be used with anew cause value to clearly identify the scenario. The MME may notretransmit the CS Service Notification to the WTRU if a response is notreceived.

The MSC/VLR may take certain actions when it receives such indicationfrom the MME. For example, the MSC/VLR may trigger the User DeterminedUser Busy procedure, or the Call Forwarding on Not Reachable procedure.

Upon reception of a CS Service Notification (with or without a call lineidentification (CLI)), the WTRU may start a timer. If no userintervention is made while the timer runs, the WTRU upon expiry of thetimer may send an ESR message to indicate that no user intervention wasreceived. The WTRU may decide to accept or reject the call based onnetwork or user configurations. If the user accepts or rejects the call,the WTRU may stop the timer and send the ESR with the appropriate userselection; (accept or reject).

Alternatively, after the timer expires, the user may not be given theoption to accept or reject a call and thus an ESR may not be sent afterthat point (and/or the WTRU may send the ESR as explained above withoutuser intervention). However, if the network side considers the MT CSFBas unsuccessful or being aborted, the network may ignore or reject anyESR that might be received from the WTRU, such as due to delayed userintervention. Thus, the MME may ignore the message.

Alternatively, the MME may reject the ESR with a Service Request messagethat may include a new reject cause to indicate that the network hasaborted the CSFB procedure. For the case where the WTRU may respond withan ESR (possibly with new type set to no response as proposed above)even without user intervention, the WTRU may not start any timer and mayconsider the procedure to be successfully completed and then enterEMM-REGISTERED.NORMAL state.

Alternatively, if the WTRU starts a timer when the CS ServiceNotification is received (as explained above) and the timer expireswithout any response from the user, the WTRU may ignore the CS ServiceNotification and therefore not send any ESR.

The described embodiments may be used in any combination and may applyto cases where user input is requested or not. Moreover, the describedembodiments may be applied to idle mode if the WTRU is in idle mode, byfirst informing the user about the call before a decision is made toaccept/reject the call. If no response is received, the discussedembodiments may be applied in that case as well.

Another problem that may occur in wireless communications systemssupporting CS and PS sessions, is that race conditions may exist betweenCSFB and intra-system HO. In the context of LTE systems, it is possiblethat there may be an occurrence of a CSFB and an intra-LTE HO event ataround the same time causing potential conflicts to arise. There areseveral scenarios that may be possible for this to occur. FIGS. 10A and10B are flow diagrams of examples of two possible scenarios, 1000A and1000B, where race conditions may occur between CSFB and intra-LTEHandover. FIG. 10A shows the scenario where the eNB is unaware of anongoing CSFB request, and FIG. 10B shows the scenario where the eNB isaware of an existing CSFB request.

FIG. 10A describes the case when the eNB is not aware of an ongoing CSFBrequest. As is shown in FIG. 10A, the WTRU sends an RRC message with aMeasurement Report to the Source eNB, 1002. Based on the RRC message,the Source eNB makes a HO decision, 1004, and sends a (X2AP) HO Requestto the Target eNB, 1006. After performing admission control, 1008, theTarget eNB sends a (X2AP) HO ACK to the Source eNB, 1010. Before theWTRU receives an RRC HO Command from the Source eNB, 1014, to perform aHO to another eNB (intra-LTE handover), the WTRU sends an ESR message tothe MME, 1012, in order to perform CSFB. This creates a race conditionbetween the CSFB request and the intra-LTE handover.

The ESR triggers a WTRU Context Modification Request message, (with theCSFB indicator), 1016, from the MME to the source eNB, which in turninitiates a UE Context Modification Failure message from the Source eNBto the MME, 1018. At that point, the source eNB may not be able toinitiate an inter-system change, (possibly with PS handover), as theWTRU is no longer under its control. Even though this problem isexplained with the assumption of X2-based HO, it is possible that theproblem occurs with the S1-based HO, or any other type of HO.

The Source eNB provides the Target eNB with information for the HO bysending a SN Status transfer message, 1020, and an UP data forwardingmessage, 1022, to the Target eNB. The WTRU sends an RRC HO Completemessage to the Target eNB, 1024, once the HO is complete. The Target eNBsends a Path Switch Request to the MME, 1026, which informs the MME ofthe completion of the intra-HO, and the MME informs the Source eNB bysending an UP End Markers Packets messages, 1028. The Source eNB stopsdata forwarding to the WTRU, 1030. The MME responds to the Target eNBwith a Path Switch Request ACK, 1032, and the Target eNB sends a Contextrelease message, 1034, to the Source eNB.

FIG. 10B shows a similar problem occurring when the eNB is aware thatthere is an ongoing CSFB request for a given WTRU. The WTRU sends an ESRmessage to the MME, 1040, in order to perform CSFB. The ESR triggers aWTRU Context Modification Request message with the CSFB indicator, 1042,from the MME to the Source eNB, which in turn initiates a UE ContextModification Response message from the Source eNB to the MME, 1044. TheSource eNB sends a configuration message, 1046, to the WTRU thatconfigures the WTRU for inter-RAT measurements. The WTRU responds to theSource eNB with an RRC LTE Measurement Report, 1048, and an RRC I-RATMeasurement Report, 1050.

The Source eNB makes an Intra-LTE HO decision, 1052, and sends an (X2AP)HO Request, 1054, to the Target eNB. The Target eNB performs admissioncontrol, 1056, and responds to the Source eNB with a (X2AP) HO ACK,1058. The Source eNB sends a HO Command to the WTRU, 1060, and the WTRUsends an RRC HO Complete message to the Target eNB, 1062. The Target eNBsends a Path Switch Request to the MME, 1064, which informs the MME ofthe completion of the intra-HO, and the MME informs the Source eNB bysending an UP End Markers Packets messages, 1066. The Source eNB stopsdata forwarding to the WTRU, 1068. The MME responds to the Target eNBwith a Path Switch Request ACK, 1070, and the Target eNB sends a Contextrelease message, 1072, to the Source eNB.

In the scenario shown in FIG. 10B, the Source eNB, even though it isaware that there is an ongoing CSFB request for the WTRU (because it hasreceived the WTRU Context Modification Request with the CSFB indicatorand has responded with a WTRU Context Modification Response), may decideto perform intra-LTE handover based on the measurement event andreports, 1048 and 1050, from the WTRU. If this did not happen, theSource eNB would have sent the Handover Required message to the MME inorder to start the inter-system change to UTRAN.

In the scenario shown in FIG. 10B, the CSFB cannot be continued by theSource eNB, and the MME is not informed about it. The MME becomesinformed about the completion of the intra-HO with the Path SwitchRequest message from the target eNB, 1064, which is transmitted to theMME from the Target eNB after the handover is complete.

The following embodiments address the problem of race conditions betweenCSFB and an intra-system HO. The embodiments apply to both X2-based HOs,S1-based HOs, or any other HO scenarios. Moreover, they also apply tothe case when the source eNB is either aware or not aware of any ongoingrequest for CSFB.

In one embodiment, the source eNB may include an indication to theTarget eNB about the CSFB request for a WTRU that is ongoing in thesource cell. The effect of this is equivalent to the target eNBreceiving the WTRU Context Modification request from the MME. The TargeteNB may start the necessary CSFB-related procedure and solicit the WTRUfor inter-RAT measurements, or send the Handover Required message to theMME. Referring back to FIG. 10A, the indication from the Source to theTarget eNB may be included in the SN Status Transfer, 1020, and may beincluded in both S1AP or X2AP messages. The Source eNB may include anymeasurement report it might have collected from a WTRU in the messagingsent to the Target eNB, such as in the Handover Request message, 1006.The Target eNB may use these measurements without configuring the WTRUagain for inter-RAT measurements.

In another embodiment, the MME may include the CSFB indicator in thePath Switch Request ACK that is sent from the MME to the Target eNB. Forexample, referring back to FIGS. 10A and 10B, the CSFB indicator may beincluded in the Path Switch Request ACK, 1032 and 1070, respectively. Assuch, the Target eNB may directly start any CSFB related procedures,such as configuring the WTRU for inter-RAT measurements or sending theHandover Required message to the MME.

According to another embodiment, the Source eNB may inform the MME whenit decides to perform, or when it actually performs, intra-system(intra-LTE) handover, given there is signaling already started for aCSFB request after the Source eNB sends the WTRU a Context ModificationResponse message. The Source eNB may inform the MME, for example, usingan S1-AP message such as the “Handover Required” message. As such, theMME may know ahead of time and may send the Context Modification Requestmessage to the Target eNB. The MME may inform other nodes, such as theSGW or PDN GW, to buffer the packets until the intra-LTE or CSFBhandover is completed.

The following embodiments describe MME-eNB interactions in order torealize CSFB.

In one embodiment, the MME may send a WTRU Context Modification request,(e.g. an S1AP message), or any other message with the purpose ofproviding information for performing CSFB for a specific WTRU. With this“CSFB information” message, the MME may inform the eNB communicatingwith the WTRU how the CSFB may be performed, which may be accomplishedvia PS HO, network assisted cell change order, cell change order, or viaRRC connection release with redirection information. The MME may informthe eNB communicating with the WTRU whether at least one SIB ofpotential CS cells may be provided to the WTRU, and/or how the CSFB maybe performed with the inclusion of new IEs in the message sent to theeNB.

Upon receiving the CSFB information message, the eNB may execute theCSFB according to the information from the MME. Thus, for CSFB foremergency calls, the MME may inform the eNB in the CSFB informationmessage that the CSFB is of high priority, and perform the CSFB via oneof the signaled methods. Referring back to FIGS. 10A and 10B, the CSFBinformation from the MME to the eNB may be included in the WTRU ContextModification Request message, 1016 and 1042, respectively, or in anyother message from the MME to the eNB.

Alternatively, in another embodiment, if no new IE may be included inthe WTRU Context Modification Request message, the high priority valueof the CSFB indicator IE in the WTRU Context Modification Requestmessage may be used as an indication to perform CSFB via RRC connectionrelease, (or network assisted cell change, or cell change order), andmay be used with System Information Blocks (SIBs) provision to the WTRU.Thus, when the CSFB indicator IE is set to “CSFB required” by the MME,the eNB may perform CSFB with PS HO. Otherwise, a value of ‘CSFB highpriority’ may cause the eNB to perform CSFB via RRC connection release,(or network assisted cell change, or cell change order), and may be usedwith SIBs provision to the WTRU.

If the MME receives a request from the WTRU or the MSC/VLR to cancel anongoing CSFB procedure, there may be options by the MME to abort theCSFB procedure. If the MME received a handover request message from theeNB, (which started a guard timer for this message), the MME may notsend a response message (i.e. HO command) to the eNB. Thus, the timer atthe eNB may ultimately expire and thereby the eNB may not autonomouslydecide to continue with the CSFB procedure.

Alternatively, the MME may send an explicit message to the eNB to informit to abort the CSFB procedure. This may be done with an existingmessage such as WTRU Context Modification Request (1016 and 1042 inFIGS. 10A, and 10B respectively). In any of these cases, the MME mayinclude a new IE to indicate the cause of the request to cancel the CSFBprocedure. Thus, the eNB when it receives this message may abort theCSFB procedure. The eNB may be configured to perform CSFB in a certainway regardless of the type ESR. This configuration may be done for aspecific time period or via explicit signaling by the MME.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

What is claimed is:
 1. A method, performed by a visitor locationregister (VLR), for avoiding an inter-system change for a wirelesstransmit/receive unit (WTRU), the method comprising: deciding to abort amobile terminating (MT) circuit switched fallback (CSFB) to the WTRU;determining if a paging request to start the MT CSFB call has been sent;and on a condition that the paging request has been sent, sending anabort message to a mobility management entity (MME) requesting the MMEto cancel the MT CSFB call.
 2. The method of claim 1 wherein deciding toabort the MT CSFB call is based on receiving a request to abort the MTCSFB call from the WTRU.
 3. The method of claim 1, further comprising:starting a timer when the paging request to the WTRU to establish the MTCSFB call is sent, wherein the deciding to abort the MT CSFB call isbased on the timer expiring before receiving a response from the WTRU.4. The method of claim 1 wherein the paging request is anSGsAP-Paging-Request message.
 5. The method of claim 1, furthercomprising: on a condition that the paging request has not been sent,aborting the MT CSFB call.
 6. A visitor location register (VLR) foravoiding an inter-system change for a wireless transmit/receive unit(WTRU), the VLR comprising: circuitry configured to decide to abort amobile terminating (MT) circuit switched fallback (CSFB) to the WTRU;circuitry configured to determine if a paging request to start the MTCSFB call has been sent; and on a condition that the paging request hasbeen sent, the VLR further comprises circuitry configured to send anabort message to a mobility management entity (MME) requesting the MMEto cancel the MT CSFB call.
 7. The VLR of claim 6, further comprising:circuitry configured to decide to abort the MT CSFB call based onreceiving a request to abort the MT CSFB call from the WTRU.
 8. The VLRof claim 6, further comprising: circuitry configured to start a timerwhen the paging request to the WTRU to establish the MT CSFB call issent, and circuitry configured to decide to abort the MT CSFB call basedon the timer expiring before receiving a response from the WTRU.
 9. TheVLR of claim 6 wherein the paging request is an SGsAP-Paging-Requestmessage.
 10. The VLR of claim 6 wherein on a condition that the pagingrequest has not been sent, the VLR further comprises circuitryconfigured to abort the MT CSFB call.
 11. A method, performed by aMobility Management Entity (MME), for avoiding an inter-system changefor a wireless transmit/receive unit (WTRU), the method comprising:receiving an abort message from a visitor location register (VLR)indicating to the MME to cancel a mobile terminating (MT) circuitswitched fallback (CSFB) call to the WTRU; on a condition that the MMEhas already received an extended service request (ESR) from the WTRU,sending a service reject message to the WTRU indicating that the MT CSFBcall is aborted; and sending no further messages to the WTRU.
 12. Themethod of claim 11, further comprising: sending no further messagespertaining to the MT CSFB call to a mobile switching center (MSC). 13.The method of claim 11, further comprising: sending an acknowledgmentmessage to the VLR including a status information element (IE)indicating a result of the MT CSFB call being canceled.
 14. The methodof claim 11, further comprising: sending an abort request to othernetwork nodes.
 15. The method of claim 11, further comprising: sending arequest to a Evolved Universal Terrestrial Radio Access Network(E-UTRAN) to abort a paging procedure.
 16. A mobility management entity(MME) for avoiding an inter-system change for a wirelesstransmit/receive unit (WTRU), the MME comprising: circuitry configuredto receive an abort message from a visitor location register (VLR)indicating to the MME to cancel a mobile terminating (MT) circuitswitched fallback (CSFB) call to the WTRU; on a condition that the MMEhas already received an extended service request (ESR) from the WTRU,circuitry configured to send a service reject message to the WTRUindicating that the MT CSFB call is aborted; and circuitry configured tosend no further messages to the WTRU.
 17. The MME of claim 16, furthercomprising: circuitry configured to send no further messages pertainingto the MT CSFB call to a mobile switching center (MSC).
 18. The MME ofclaim 16, further comprising: circuitry configured to send anacknowledgment message to the VLR including a status information element(IE) indicating a result of the MT CSFB call being canceled.
 19. The MMEof claim 16, further comprising: circuitry configured to send an abortrequest to other network nodes.
 20. The MME of claim 16, furthercomprising: circuitry configured to send a request to a EvolvedUniversal Terrestrial Radio Access Network (E-UTRAN) to abort a pagingprocedure.